Transmission device for land vehicle, such as a cart

ABSTRACT

For a first gear ratio, the flyweights engage the clutch and thus connect the input to the sun wheel, whilst the ring gear is immobilised by the free wheel. For the second gear ratio, the flyweights push the ring gear against the axial thrust of the helical teeth in a direction causing the engagement of the clutch against the ring and thus, at the same time, the disengagement of the clutch. The thrust produced by the teeth on the ring gear decreases and transmission then occurs from the input to the ring gear, whilst the sun wheel is immobilised by its free wheel. The speed increases further, the flyweights rise further, compress the prestressed springs and push the ring in the direction of the engagement of the clutch, which produces direct drive.

This invention relates to a transmission device for land vehicles, andin particular for small automotive vehicles, such as “unlicensed”lower-category cars. Light vehicles with engines with reduced capacityand/or power and/or with a limited maximum speed, which, depending onnational legislation, can in certain countries be driven by people whodo not have a driving license, are known as lower-category cars.

The invention relates more particularly to a transmission device havinga particularly simple structure relative to the number of gear ratiosavailable.

Almost all An automatic transmission devices use differential mechanismsand in particular planetary gear trains in which selective couplingmeans such as brakes, clutches and/or free wheels are used to change thegear ratio supplied by each set of planetary gears. Conventionally, aplanetary gear train provides one of two gears, one of which is a directdrive gear obtained using a clutch that couples together two intermeshedrotational elements on the gear train. There are planetary gear trainsthat supply more than two gears, but in this case these are generally“complex” planetary gear trains, i.e. with more than three intermeshedrotational elements, which are in fact equivalent to at least two setsof planetary gears. Consequently, to produce a three-gear transmissiondevice, two sets of planetary gears are currently required.

Furthermore, An automatic control is particularly complex and costly toachieve.

EP-A-0 683 877 relates to an automatic transmission device on which theAn automatic control is simplified through the use of the axial thrustof the helical teeth, both to measure the torque transmitted and as anactuating force in proportion to this torque. This force holds open adirect-drive clutch between the input and the output of the planetarygear train when the planetary gear train is operating as a reductionunit. During this time, the third element of the gear train isimmobilised by a free wheel. The direct-drive clutch is closed under theaction of centrifugal flyweights when the speed of rotation issufficient to enable them to overcome the axial thrust of the teeth.

With this known device, control is certainly simplified and consumesless energy, but again a simple planetary gear train only provides twogears.

Although a planetary gear train can in theory provide a number of gearratios, it has not been possible in practice to obtain more than two,given the complexity involved in changing between two gears.

The object of the present invention is to propose a transmission devicein which the means required to change from one gear to the other in adifferential mechanism are considerably simplified.

Another object of the present invention is to propose a transmissiondevice in which a simple differential mechanism, i.e. with only threeintermeshed elements, is able to provide more than two gear ratios.

A further object of the present invention is to propose a transmissiondevice that makes a number of gears available with a particularly simplestructure and improved mechanical efficiency.

An additional object is to propose a particularly light transmissiondevice that is economical to produce.

According to the invention, to meet all or some of these objects, the Anautomatic transmission device, particularly for land vehicles,comprises:

-   -   a rotational input element and a rotational output element;    -   a first and second toothed element, which each mesh with a third        toothed element;    -   first and second coupling means to selectively couple a first        one of the rotational input and output elements with the first        and second toothed elements respectively;    -   a third coupling means to selectively couple the second of the        rotational input and output elements with the third toothed        element;    -   first and second stopping means to selectively immobilise the        first and second toothed elements respectively;    -   a first actuator for the first coupling means; and    -   a second actuator capable of a first position in which the first        coupling means is capable of an engaged state and a disengaged        state, and the second coupling means is disengaged, and a second        position in which the first coupling means is disengaged and the        second coupling means is in an engaged state.

The transmission device according to the invention allows for threegears to be obtained easily with a single planetary gear train. Forthis, the first rotational element, which is typically the rotationalinput element (for example an input shaft), is coupled by the firstcoupling means with the first toothed element, or by the second couplingmeans with the second element, or simultaneously by the first and secondcoupling means with the first and second toothed elements, in such a wayas to achieve direct drive.

A fourth state, in which the first and second coupling means aredisengaged, may correspond to neutral.

In its first position, the second actuator provided for according to theinvention allows for the selection of either neutral or the first gearratio, by means of the first actuator, and its second positioncorresponds to the second gear ratio, in which the first coupling meansis disengaged and the second coupling means is in an engaged state.

Preferably, in the second position, to ensure that the first couplingmeans is disengaged, the second actuator exerts thrust on the firstactuator through the intermediary of the first coupling means. Thus, theforce applied to the second actuator is used both to place the secondcoupling means in an engaged state and to act on the first actuator inthe direction of disengagement of the first coupling means.

In a preferred embodiment of the invention, the second actuator iscapable of a third position, in which the first and second couplingmeans are in an engaged state.

This function is preferably achieved by moving the second actuatorbeyond its second position so that it acts on a third actuator thatpushes the first coupling means against the first actuator, which meetsa stop and then acts as a counterbearing.

At the same time, it is preferable that the first actuator be associatedwith a first control means suitable for the gradual setting in motion ofthe vehicle. This eliminates an input clutch between the engine and thetransmission device. This represents a significant saving in terms ofweight, cost and size. This first control means is preferablycentrifugal. It may act on the first coupling means against a returnspring, but this is not vital. On the contrary, a certain “drag” in thefirst coupling means may be preferred when the engine is at its idlingspeed, which allows the vehicle to be moved forward at very low speedswhen the engine is idling. The driver can prevent this very slowmovement by activating the brake pedal.

The first and second stopping means are preferably free wheels (alsoknown as “one-way clutches”), which have the advantage that they do notrequire any specific control.

Finally, reversing means may be provided that, when they are activated,directly or indirectly hold the second actuator in its first position.This prevents the transmission device from establishing high gear ratiosduring operation in reverse.

Other features and advantages of the invention will furthermore emergefrom the following description, relating to a non-limitative example.

In the appended drawings:

FIG. 1 is an axial cross-section half-view of an An automatictransmission device according to the invention, in neutral;

FIG. 2 is an axial cross-section of a device according to FIG. 1 (exceptthat the reversing means are no longer shown), operating in first gear;and

FIGS. 3 and 4 are half-views analogous to the upper part of FIG. 2, butwhen the device is operating in second and third gears respectively.

In the example shown in FIG. 1, the transmission device is mainlyconstituted by a differential mechanism 1 comprising:

-   -   a housing element 2, which is only partly shown and comprises in        particular a stator shaft 21, non-translatable and        non-rotatable, extending along the general X-axis of the        mechanism;    -   a rotational input element 3, non-translatable relative to the        housing element 2, and comprising in the example shown a disc        centred on the general X-axis beyond one end of the stator shaft        21, the said disc being intended to be connected directly or        indirectly, in a way not shown, to the shaft of a vehicle        driving engine;    -   a rotational output element 4, intended to be connected at least        indirectly to the vehicle's wheels;    -   a rotational sun element 5, arranged along the X-axis around the        stator shaft 21 and able to rotate relative to it around the        X-axis;    -   a rotational element forming a ring gear 6, fitted around the        X-axis and arranged around the sun element 5 and the stator        shaft 21;    -   a rotational element forming a planet carrier 7, carrying,        evenly distributed around the X-axis, journals 71 that are        eccentric relative to the general X-axis, on which planet        pinions 72 rotate freely and mesh both with the sun element 5        and the ring gear 6, in such a way as to form a planetary gear        train with them;    -   a first free wheel 8, which prevents the sun element 5 from        rotating in the opposite direction to the input element 3        relative to the housing element 2;    -   a second free wheel 9, which prevents the ring gear 6 from        rotating in the opposite direction to the input element 3        relative to the housing element 2;    -   a first friction coupling means 11, selectively coupling the        input element 3 with the sun element 5 in rotation around the        X-axis;    -   a second friction coupling means 12, selectively coupling the        input element 3 and the ring gear 6 in rotation around the        X-axis;

The friction coupling means 11 and 12 are wet multidisc clutches. Toform them, the input element 3 has a series of ribs 31 that form a crownaround the X-axis, being positioned axially. The crown of ribs 31surrounds outer splines 51 that are attached to the sun wheel 5, and issurrounded by inner splines 61 that are attached to the ring gear 6. Thefirst friction coupling means 11 comprises a stack of discs that arealternately coupled in rotation with the ribs 31 and the splines 51. Thesecond friction coupling means 12 comprises a stack of discs that arealternately coupled in rotation with the ribs 31 and the splines 61.

The transmission device also has a first actuator 111 formed by a ringcentred around the X-axis, which has openings 112 through which the ribs31 extend. The first actuator 111 is fitted axially and slides on theribs 31.

A first control means 113 is also provided for in the form of a seriesof centrifugal flyweights 118 distributed around the X-axis and drivenin rotation around the X-axis by the rotational input element 3. Underthe effect of the centrifugal force developed by the rotation of theinput element 3 around the X-axis, the flyweights can pivot from theidle position shown to a raised position that can be seen in FIG. 2.When the flyweights 118 are in idle position, the first actuator 111 canitself be in idle position resting against the radial surface of theelement 3, from which the ribs 31 start. On the other hand, when theflyweights 118 are in the raised position shown in FIG. 2, the firstactuator 111 is pushed against the stack of discs of the first couplingmeans 11 by a projection 114 of each flyweight.

The transmission device also has a second actuator constituted by a cup62 that covers the coupling means 11 and 12 on the opposite side to thefirst actuator 111. The cup 62 is attached to the ring gear 6, which canslide parallel to the X-axis. The gear teeth 63 of the ring gear 6 arearranged axially and are sufficiently long that they mesh completelywith the planet pinions 72 whatever the axial position of the ring gear6. The second actuator 62 also has a ring 122 with teeth around itsouter edge that are engaged in the splines 61. A compression spring 121is interposed between the ring 122 and a face 69 that is attached to thering gear 6. The spring 121 pushes the ring 122 back towards the discsand the first actuator 111. The teeth on the ring 122 are radiallylonger than the teeth on the discs attached to the ring gear 6. When thespring 121 is in idle state, it pushes the ring 122 until the extendedteeth of the ring 122 rest against a shoulder 64 on the ring gear 6under a predetermined prestress. The shoulder 64 forms a transitionbetween a deeper area of the splines 61, in which the ring 122 is heldaxially, and a shallower area that can receive the teeth of the discs ofthe coupling means 12.

The transmission device also has a third control member 116 constitutedby a sliding ring fitted on the radially inner side of the ribs 31, onthe side axially opposite to the first actuator 111 relative to thestack of discs of the first friction coupling means 11. The thirdactuator 116 is prevented from leaving the ribs 31 by a stop bush 117secured onto the ribs 31. From the limit position defined by the stopbush 117, the third control member 116 can slide on the ribs 31 to pushthe stack of discs of the first coupling means 11 towards the firstactuator 111.

The transmission device also has a second control means 13, which hascentrifugal flyweights 131 that are driven in rotation around the X-axisby the planet-carrier 7, and therefore at a speed proportional to thespeed of rotation of the output element 4. The flyweights 131 have aprojection 132 that rests against an end face 66 of the ring gear 6through the intermediary of a thrust bearing 133 as the speed ofrotation of the planet carrier 7, and therefore of the flyweights 131,around the X-axis is generally different from the speed of the ring gear6.

When the flyweights 131 are idle (as shown in FIG. 1), the distancebetween the ring 122 resting against the shoulder 64 and the firstcontrol member 111 is such that the second friction coupling means 12 isin a disengaged state. When the flyweights 131 rise up under the actionof the centrifugal force, as shown in FIGS. 3 and 4, which are describedin detail below, the ring gear 6 and in particular the second actuator,comprising the cup 62, are pushed towards the first actuator 111 andtherefore in the direction tending to compress the stack of discs of thesecond friction coupling means 12.

The teeth on the planetary gear train formed by the sun wheel 5, thering gear 6 and the planet pinions 72 mounted in rotation on theirplanet carrier 7 are helical and the direction of inclination of theteeth is selected so that the axial reaction originating in the teeth 63of the ring gear 6 when a propelling torque is transmitted, is in thedirection tending to release the second friction coupling means 12 andpush down the flyweights 131. This axial thrust is represented by thearrow P_(A) in FIG. 2.

In the example shown in FIG. 1, the transmission device is also equippedwith a reversing system 14 comprising a dog clutch 141 that is coupledin rotation around the X-axis with the planet carrier 7, but which canalso slide axially relative to the planet carrier 7 by means of manualcontrol means, not shown. To enable this sliding relative to the planetcarrier 7, which is immobilised axially by bearings 73 on the statorshaft 21, the planet carrier and the dog clutch 141 are equipped withintermeshed splines 74, 142. On its radially outer face, the dog clutch141 has teeth 143 which, in the forward position shown with solid linesin FIG. 1, mesh with corresponding teeth 41 on the output 4. In thereverse position shown partly with a dotted line, the teeth 143 meshwith a reverse idler gear 42, which in turn meshes with a second set ofteeth 43 on the output 4. Furthermore, the dog clutch 141 has a stoparea 144 which, when the dog clutch 141 is in reverse position, isplaced around the flyweights 131, as shown with dotted lines in FIG. 1,preventing them from leaving their lowered idle position. As will beseen more clearly below, this prevents the transmission device fromoperating other than in neutral or in its first gear ratio when the dogclutch 141 is in reverse position.

The operation of the transmission device will now be described in moredetail.

In the situation shown in FIG. 1, the rotational input element 3 isstationary, so that the flyweights 118 are lowered. The flyweights 131of the second control means 13 are also lowered. The two frictioncoupling means 11 and 12 are in a disengaged state so that therotational input element 3 can rotate freely relative to the outputelement 4, which corresponds to neutral. However, the vehicle isprevented from moving in reverse, for example along a sloping street, ifthe parking brake is insufficiently applied, as the planet carrier 7could not rotate in the reverse direction without causing the sun wheel5 and/or the ring gear 6 to rotate in the same direction; neither thesun wheel 5 nor the ring gear 6 can rotate as described as they areprevented from doing so by the free wheels 8 and 9 respectively. This isbeneficial for hill starts, which are generally a difficult exercise fordrivers of lower-category cars, who sometimes have limited drivingexpertise.

Similarly, if the dog clutch 141 is pushed into reverse position, thevehicle is, for similar reasons to those described above, prevented frommoving forwards. This is beneficial for hill start operations inreverse.

From this situation, if the driver starts the engine, the rotationalinput element 3, coupled directly or indirectly (i.e. through theintermediary of a belt or a gearing system) but permanently to thevehicle's engine, starts to rotate and the flyweights of the firstcontrol means 113 rise as shown in FIG. 2; this tends to compress thediscs of the first coupling means 11 against the third actuator 116 butwithout the radially outer part of the first actuator 111 pressingagainst the stack of discs of the second friction coupling means 12.When the rotational input element 3 rotates around the X-axis at a speedof rotation corresponding to the engine idling speed, the centrifugalforce in the flyweights is so low that there is merely “drag” in thefirst friction coupling means 11, which may possibly cause a certainamount of rotation in the sun wheel 5. If the driver of the vehiclecauses an increase in the speed of rotation of the engine, thecentrifugal force in the flyweights on the first control means 113gradually increases and the increasing engagement of the first frictioncoupling means 11 gradually brings the rotational input element 3 andthe sun wheel 5 together in rotation. As the planet carrier 7 tends tobe immobilised by the load to be driven, the rotation of the sun element5 tends to cause a reverse rotation in the ring gear 6, but this isprevented by the second free wheel 9 so that the ring gear 6 isimmobilised and the planet carrier 7 is driven in rotation in the samedirection as the sun element 5 but at a much lower speed than the sunelement 5.

At the same time, the speed of rotation of the output element 7 does notgenerate sufficient centrifugal force in the flyweights 131 to overcomethe opposing action of the axial thrust P_(A). This operationcorresponds to the first gear ratio.

If the speed of rotation of the planet carrier 7 increases, theflyweights 131 eventually rise as shown in FIG. 3 and push the ring gear6 in the direction of the compression of the second friction couplingmeans 12 against the first actuator 111. This force is less than theprestressing force of the springs 121 so that the ring 122 restingagainst the shoulder 64 moves as a single unit with the cup 62 attachedto the ring gear 6.

At the start of this movement, the excess axial force produced by theflyweights 131 relative to the axial thrust P_(A) is very low, justsufficient to produce drag in the friction coupling means 12. However,this drag decreases the force transmitted by the sun element 5 andtherefore decreases the axial thrust P_(A). Thus, the axial force in thedirection of the engagement of the friction coupling means 12 graduallybecomes greater and greater until, as shown in FIG. 3, the frictioncoupling means 12 is not only pushed up against the first actuator 111,but is eventually pushed into abutment against the disc of therotational input element 3, whereby the first friction coupling means 11is released. Consequently, the movement of the rotational input element3 is no longer transmitted to the sun element 5, but to the ring gear 6.The sun element 5 then tends to rotate in reverse, but is prevented fromdoing so by its free wheel 8, so that the planet carrier 7 is driven ata new speed of rotation that is greater relative to the speed of theinput element 3 than during operation in the first gear.

In fact, as is always the case when changing to a higher gear ratio in avehicle, it is the speed of the input element 3 that decreases to matchthe new gear ratio, as the inertia of the load formed by the movingvehicle works against a significant change in the speed of rotation ofthe output (therefore of the planet carrier 7 in the example) during thegear changing process. This decrease in the speed of the rotationalinput element 3 during the gear changing process reduces the centrifugalforce in the flyweights 118 and therefore favours the lowering of theflyweights 118 through the thrust exerted by the second actuator 62 viathe second friction coupling means 12 on the first actuator 111.

If the speed of the output and consequently the speed of the planetcarrier 7 continues to increase, the flyweights 131 eventually rise morethan in the situation shown in FIG. 3, as shown in FIG. 4, whereby thecup 62 is pushed further to the left, due to the strength threshold ofthe spring 121 being exceeded and the spring 121 being then compressed.The ring 122 moves away from the shoulder 64 and a bearing face 67 onthe cup 62 pushes the third actuator 116 towards the first actuator 111,with compression of the first friction coupling means 11. Onceinitiated, this movement can only intensify as it corresponds to afurther drop in thrust P_(A). In fact, some of the torque is thentransmitted by the sun wheel 5, whereby the part transmitted by theteeth of the ring gear 6 is correspondingly reduced.

In this new situation, the two friction coupling means 11 and 12 are inan engaged state, and couple together the input element 3, the ring gear6 and the sun element 5. This is a direct drive situation. The planetcarrier 7 is also rotating at the common speed of rotation of the inputelement 3, the sun element 5 and the ring gear 6. The flyweights 118 onthe first control means 113 remain in their lowered state.

During the various gear changing processes in the direction of reducingthe speed of the input element 3 relative to the speed of the planetcarrier 7, the gradual reduction, during each changing process, of theaxial thrust, which tends to oppose this change, has the effect ofstabilising the new gear ratio as soon as it has started to beestablished. Consequently, for the same engine torque, down-shifting canonly take place at a lower speed of rotation of the planet carrier 7.

Down-shifting take place as follows:

-   -   from third to second gear, the change is initiated when slip        starts to occur in the first friction coupling means 11 when the        force generated by the flyweights 131 is only just sufficient to        both overcome the thrust P_(A) and compress the springs 121.        This slip causes the sun element 5 to slow down until it is        immobilised by its free wheel 8. This immobilisation increases        the axial thrust P_(A) and the flyweights 131 are thus pushed        into their position as shown in FIG. 3, with the spring 121        relaxing to its minimum prestressed position.    -   from this situation, if the force of the flyweights 131 drops        further and becomes insufficiently greater than the axial thrust        P_(A) to sufficiently compress the second friction coupling        means 12 with regard to the engine torque to be transmitted        between the input element 3 and the ring gear 6, the second        friction coupling means 12 starts to slip, until it is        immobilised by its free wheel 9. This tends to cause racing in        the input element 3, and therefore an increase in the        centrifugal force produced by the flyweights 118; the first        friction coupling means 11 is thus returned to an engaged state        to produce operation in first gear. The axial thrust P_(A)        increases and completely pushes down the flyweights 131, to end        in the situation shown in FIG. 2.

The transmission device described above has the advantage, despite itsgreat simplicity, of featuring the so-called “kick-down”, which enablesthe driver to change into a lower gear simply by pressing theaccelerator pedal, thanks to the resulting increase in the thrust P_(A).

When the planet carrier 7 tends to rotate more quickly than the inputelement 3 (operation in engine braking mode), i.e. when the driver ofthe vehicle releases the accelerator pedal or when the vehicle is movingdownhill, the torque applied to the input element 3 becomes negative,the axial thrust P_(A) reverses and consequently assists the flyweights131 to compress the second friction coupling means 12 and even, most ofthe time, the springs 121, and therefore also the first frictioncoupling means 11, to achieve direct drive operation.

For reasons of simplicity, FIGS. 2 to 4 do not show the reversing device14 and instead show a simple output gearing 76, attached to the planetcarrier 7.

The invention is not of course limited to the examples described andshown.

Other return springs can be envisaged, for example a spring tending toseparate the first and third actuators 111, 116 from each other, or moregenerally to disengage the first friction coupling means 11.

Reverse could be obtained in a different way to that described, forexample by immobilising the planet carrier 7 and activating a dog clutchthat would release the ring gear 6 from the free wheel 9 and connect thering gear 6 to the output shaft.

The arrangement described, with the two friction coupling means 11 and12 arranged one around the other, is not limitative and they could, forexample, be aligned axially.

1. An automatic transmission device, comprising: a rotational inputelement and a rotational output element; a first and a second toothedelement that each mesh with a third toothed element; a first couplingmeans to selectively couple a first one of the rotational input andoutput elements with the first toothed elements; a second coupling meansfor selective coupling said first one of the rotational input and outputelements with said second toothed element; a third coupling means tocouple a second one of the two rotational input and output elements withthe third toothed element a first and second stopping means toselectively immobilize the first and second toothed elementsrespectively; a first actuator for the first coupling means; andcharacterized by a second actuator having a first position in which thefirst coupling means is either in an engaged state or a disengagedstate, and the second coupling means is disengaged, and a secondposition in which the first coupling means is disengaged and the secondcoupling means is in an engaged state, wherein in the second position,the second actuator exerts thrust on the first actuator in the directionof the disengagement of the first coupling means.
 2. An automatictransmission device according to claims 1, wherein a third actuator isfitted to selectively place the first coupling means in an engaged stateby pushing the first coupling means against the first actuator.
 3. Anautomatic transmission device according to claim 1, wherein there isprovided for the second actuating means a second control means withcentrifugal flyweights sensitive to the speed of rotation of the thirdtoothed element and/or the output element.
 4. An automatic transmissiondevice according to claim 1, wherein at least one of the first andsecond stopping means is a free wheel.
 5. An automatic transmissiondevice according to claim 1, wherein the first toothed element is a sunwheel, the second toothed element is a ring gear and the third toothedelement is a planet carrier on which the planet pinions mesh with thesun wheel and the ring gear.
 6. An automatic transmission deviceaccording to claim 1, wherein reversing means are provided which, atleast indirectly, selectively hold the second actuator in its firstposition.
 7. An automatic transmission device according to claim 1,wherein said thrust is exerted by the intermediary of the secondcoupling means.
 8. An automatic transmission device according to claim7, wherein the first actuator is at the same time a counter bearing isat the same time a counterbearing for the second coupling means in anengaged state.
 9. An automatic transmission device according to claim 1,wherein there is provided for the first actuator a first control meansfor the gradual setting in motion of the vehicle.
 10. An automatictransmission device according to claim 9, wherein the first controlmeans is of centrifugal type.
 11. An automatic transmission deviceaccording to claim 10, wherein the first control means has centrifugalflyweights sensitive to the speed of the rotational input element andtending to put the first coupling means in an engaged state.
 12. Anautomatic transmission device according to claim 1, wherein the secondactuator is capable of a third position in which the first and secondcoupling means are in an engaged state.
 13. An automatic transmissiondevice according to claim 12, wherein in its third position, the secondactuator acts on a third actuator, which pushes the first coupling meansagainst the first actuator.
 14. An automatic transmission deviceaccording to claim 12, wherein a prestressed spring is provided betweenthe second actuator and the second coupling means.
 15. An automatictransmission device according to claim 12, wherein a prestressed springis fitted to define a strength threshold that the second actuator mustexceed to move from its second to its third position.
 16. An automatictransmission device according to claim 1, wherein means are provided toapply thrust produced by teeth to the second actuator from second tofirst position.
 17. An automatic transmission device according to claim16, wherein the second actuator is attached to the second toothedelement.
 18. An automatic transmission device comprising: a rotationalinput element and a rotational output element; a first and a secondtoothed element that each mesh with a third toothed element; a firstcoupling means to selectively couple a first one of the rotational inputand output elements with the first toothed element respectively; asecond coupling means for selectively coupling said first one of therotational input and output elements with said second toothed elements;a third coupling means to couple the second of the two rotational inputand output elements with the third toothed element first and secondstopping means to selectively immobilize the first and second toothedelements respectively; a first actuator for the first coupling means; asecond actuator having a first position in which the first couplingmeans is either in an engaged state or a disengaged state, and thesecond coupling means is disengaged, and a second position in which thefirst coupling means is disengaged and the second coupling means is inan engaged state, and a third actuator is fitted to selectively placethe first coupling means in an engaged state by pushing the firstcoupling means against the first actuator.
 19. An automatic transmissiondevice comprising: a rotational input element and a rotational outputelement; a first and a second toothed element that each mesh with athird toothed element; a first coupling means to selectively couple afirst one of the rotational input and output elements with the firsttoothed element respectively; a second coupling means for selectivelycoupling said first one of the rotational input and output elements withsaid second toothed elements; a third coupling means to couple thesecond of the two rotational input and output elements with the thirdtoothed element first and second stopping means to selectivelyimmobilize the first and second toothed elements respectively; a firstactuator for the first coupling means; and characterized by a secondactuator having a first position in which the first coupling means iseither in an engaged state or a disengaged state, and the secondcoupling means is disengaged, and a second position in which the firstcoupling means is disengaged and the second coupling means is in anengaged state, wherein there is provided for the second actuator asecond control means with centrifugal flyweights sensitive to the speedof rotation of the third toothed element and/or the output element. 20.An automatic transmission device comprising: a rotational input elementand a rotational output element; a first and a second toothed elementthat each mesh with a third toothed element; a first coupling means toselectively couple a first one of the rotational input and outputelements with the first toothed element respectively; a second couplingmeans for selectively coupling said first one of the rotational inputand output elements with said second toothed elements; a third couplingmeans to couple the second of the two rotational input and outputelements with the third toothed element first and second stopping meansto selectively immobilize the first and second toothed elementsrespectively; a first actuator for the first coupling means; andcharacterized by a second actuator having of a first position in whichthe first coupling means is either in an engaged state or a disengagedstate, and the second coupling means is disengaged, and a secondposition in which the first coupling means is disengaged and the secondcoupling means is in an engaged state wherein means are provided toapply thrust produced by teeth to the second actuator in the directionof the movement of the second actuator from second to first position.21. An automatic transmission device comprising: a rotational inputelement and a rotational output element; a first and a second toothedelement that each mesh with a third toothed element; a first couplingmeans to selectively couple a first one of the rotation input and outputelements with the first toothed element respectively; a second couplingmeans for selectively coupling said first one of the rotational inputand output elements with said second toothed elements; a third couplingmeans to couple the second of the two rotational input and outputelements with the third toothed element first and second stopping meansto selectively immobilize the first and second toothed elementsrespectively; a first actuator for the first coupling means; andcharacterized by a second actuator having a first position in which thefirst coupling means is either in an engaged state or a disengagedstate, and the second coupling means is disengaged, and a secondposition in which the first coupling means is disengaged and the secondcoupling means is in an engaged state, wherein there is provided for thefirst actuator a first control means for the gradual setting in motionof the vehicle.
 22. An automatic transmission device according to claim21, wherein the first control means has centrifugal flyweights sensitiveto the speed of the rotational input element and tending to put thefirst coupling means in an engaged state.
 23. An automatic transmissiondevice comprising: a rotational input element and a rotational outputelement; a first and a second toothed element that each mesh with athird toothed element; a first coupling means to selectively couple afirst one of the rotational input and output elements with the firsttoothed element; a second coupling means for selectively coupling saidfirst one of the rotational input and output elements with said secondtoothed elements; a third coupling means to couple the second of the tworotational input and output elements with the third toothed elementfirst and second stopping means to selectively immobilize the first andsecond toothed elements respectively; a first actuator for the firstcoupling means; and characterized by a second actuator having of a firstposition in which the first coupling means is either in an engaged stateor an disengaged state, and the second coupling means is disengaged, anda second position in which the first coupling means is disengaged andthe second coupling means is in an engaged state, wherein the secondactuator has a third position in which the first and second couplingmeans are in an engaged state.
 24. A transmission device according toclaim 23, wherein in its third position, the second actuator acts on athird actuator, which pushes the first coupling means against the firstactuator.
 25. A transmission device according to claim 23, wherein aprestressed spring is fitted to define a strength threshold that thesecond actuator must exceed to move from its second to its thirdposition.